1. Field of the Invention
The present invention relates to two-stroke internal combustion engines comprising one pair or a plurality of pairs of scavenging passages that adopt a reverse scavenging system, and more specifically to two-stroke internal combustion engines that are capable of suppressing the short-circuiting of fresh charge (unburnt air-fuel mixture), while at the same time also being capable of improving scavenging efficiency, combustion efficiency, and the like.
2. Background Art
Ordinarily, in standard two-stroke gasoline engines conventionally used in portable powered work machines, such as lawn mowers, chainsaws, etc., a spark plug is disposed at a head portion of a cylinder. An intake port, a scavenging port, and an exhaust port that are opened/closed by a piston are formed in a barrel portion of the cylinder. There are no independent strokes dedicated to intake and exhaust alone. And one cycle of the engine is completed with two strokes of the piston.
More specifically, by an up-stroke of the piston, an air-fuel mixture is drawn into a crankchamber below the piston from the intake port, while the air-fuel mixture is pre-pressurized by a down-stroke of the piston and the pre-pressurized air-fuel mixture is blown out from the scavenging port into a combustion actuating chamber above the piston, thereby exhausting the combustion waste gas to the exhaust port. In other words, the scavenging of the combustion waste gas is performed utilizing the gas flow of the air-fuel mixture.
For this reason, an unburnt air-fuel mixture often becomes mixed in the combustion waste gas (exhaust gas), the amount of fresh charge (unburnt air-fuel mixture) that is exhausted into the atmosphere without being used for combustion, that is, the so-called short circuited amount, is large, and fuel economy is inferior as compared to four-stroke engines. Further, HC (unburnt components of the fuel), CO (incomplete combustion components of the fuel), etc., which are noxious components, are contained in the exhaust gas in large amounts. Therefore, while the machines may be small in size, environmental pollution still is a concern, and there are such issues as how to accommodate emission regulations as well as demands for improved fuel economy, which are bound to become even more stringent in the years to come.
In view of such issues, various improvements have hitherto been proposed with regard to the shape and structure of scavenging passages as can be seen in Patent Documents 1 and 2 cited below, for example.
In addition, with respect to a two-stroke internal combustion engine comprising one pair or a plurality of pairs of scavenging passages that adopt a reverse scavenging system (Schnürle-scavenging system) in such a manner as to communicate a combustion actuating chamber formed above a piston with a crankchamber, the present applicant has also previously proposed the forming of, in a planar member (gasket) fitted between a cylinder, into which the piston is fitted and inserted, and the crankcase, throttling holes or throttling cutout openings of fixed opening areas that are smaller than the sectional areas of the scavenging passages in order to throttle the vicinity of inlets of the scavenging passages as disclosed in Patent Document 3 cited below.
According to this proposal, since the throttling holes are provided near the scavenging inlets, the pressure difference between the crankchamber and a point in the scavenging passages downstream of the throttling holes becomes greater as compared to a case where no throttling holes are provided, and the air-fuel mixture of the crankchamber bursts out from the throttling holes at once and flow downstream thereof. In other words, the pressure and flow speed of the scavenging gas are increased as compared to a case where the vicinity of the scavenging inlets of the scavenging passages is not throttled, and the scavenging gas that has passed through the throttling holes is blown out into the combustion actuating chamber from scavenging outlets while expanding rapidly and generating a predetermined turbulence.
Thus, the atomization of fuel is facilitated, scavenging efficiency (trapping efficiency) improves, while at the same time combustion efficiency also improves. Consequently, the desired output is obtained with less fuel, the noxious components within the exhaust gas, THC [=total amount of unburnt gas components such as HC (hydrocarbon) and the like] in particular, can be reduced effectively and, further, fuel economy improves as well.    [Patent Document 1] JP Patent Publication (Kokai) No. 2008-274804 A    [Patent Document 2] JP Patent Publication (Kokai) No. 11-315722 A (1999)    [Patent Document 3] JP Patent No. 4082868